I am trying to diagnose a problem while away from the boat, as our daughter tries not to panic in one of her first "in command" experiences.

We have a dual battery installation, with a wet-cell starting battery (Battery 1 on the OFF-1-BOTH-2 switch) and 2 Group 31 AGM's in parallel as the House battery (#2). After running 4-5 hours under power with the switch on BOTH, she was able to sail for a few hours. When she then tried to start the engine using only the starting battery, the starter motor responded but couldn't turn over the engine, as it routinely could prior to yesterday. Fortunately, the House bank handled the task easily. There is an installed voltage meter that will test either battery bank. After anchoring and shutting everything down for the night, and switching the battery switch to OFF,both banks returned 13 volts.

I probably should add that (new this season), a 100 watt solar panel feeds the House bank directly from a controller. With the battery switch on BOTH, the solar panel also feeds the starting battery through the switch. Although I don't leave the switch on BOTH when not aboard, I have found that the solar panel keeps the House bank fully charged, and that it tops up the starting batt quickly on a bright day. I judge the state of charge only by when the controller shuts down, corroborated by the voltage meter--an imperfect measure for sure. I don't think the new solar installation has anything to do with the immediate problem.

From all of the above, I believe that we are not dealing with a dead starting battery. I will get a reading from the voltage meter this morning after the batteries have rested overnight. This is at most the third season for the starting battery. I am suspecting a bad connection to the starting battery, but having difficulty envisioning how that could be. Does anyone think I am heading in the wrong direction, or have other suggestions for diagnosing in absentia?